This invention relates to a superposition drive for a superimposed steering system of a motor vehicle.
From the prior art, steering wheels which realize a superimposed steering system are known. Such steering wheels include a superposition drive for generating a steering angle which is superimposed with a steering angle generated by the driver by actuating the steering wheel rim of the steering wheel, so that a rotary movement of a steering shaft connected with the steering wheel is composed of the steering angle generated by the driver and the steering angle generated by the superposition drive
Due to manufacture, the parts of the superimposed steering system have certain tolerances whose minimization involves considerable costs. During assembly of the individual parts, these tolerances add up, which results in deviations in the engagement of transmission components of the superposition drive. The cooperation of the transmission components also can change during operation of the superimposed steering system, in particular in dependence on the operating and environmental conditions acting on the superposition drive (e.g. direction of rotation and its reversal, wear, temperature, air humidity). A change of the cooperation of the transmission components in turn can influence a development of noise caused by the superposition drive. This fact can be particularly critical when the superimposed steering system is to be used in the direct hearing range of a person, as it is the case when it is applied in the steering wheel of a motor vehicle.
From DE 10 2006 013 288 A1 a superposition apparatus for a superimposed steering system with an actuating device (electric motor) is known, which together with a worm shaft non-rotatably connected with the same is pivotally mounted on a transmission housing. By a biasing device engaging the end of the worm shaft facing away from the actuating device, which includes a coil spring under tension, the worm shaft is pulled against a worm gear and in this way the clearance between the tooth flanks thereof is brought to zero.
It may be a disadvantage of such an arrangement that upon occurrence of a force which acts against the force applied by the biasing device and exceeds the same, worm gear and worm shaft can be pressed apart (moved away from each other) and the clearance between the tooth flanks can exceed a critical measure.